future proof the last_successful_NAME_ami to give a duplicate regionalised url

Currently the ami endpoint doesn't specify a region; you just have to assume
it is us-east-1. This commit adds a second, permanent url, that includes
the region information.

The former url last_successful_#{stemcell_S3_name}_ami can be deprecated.
Merge pull request #136 from drnic/ami_us_east_1_endpoint

future proof the last_successful_NAME_ami to give a duplicate regionalised url